1. Field of the Invention
The present invention relates to a nonmonocrystalline photoelectric conversion element having an improved lower conductive layer surface configuration, i-layer crystal structure, and doped layer structure.
2. Related Background Art
Increase in photoelectric conversion efficiency and improvement in optical degradation have been studied heretofore for the photoelectric conversion elements incorporating the pin junction of a nonmonocrystalline semiconductor.
It is known that increasing a concentration of dopant in a doped layer decreases activation energy of the doped layer, thereby increasing the built-in potential of the pin junction and the open-circuit voltage of the element.
It is also known that use of a microcrystalline material for the i-type semiconductor layer improves optical degradation.
It is reported that a solar cell using microcrystalline silicon (.mu. c-Si) achieved a photoelectric conversion efficiency of 4.6% using plasma enhanced CVD using VHF (70 MHz) and that the solar cell demonstrated no optical degradation at all, as seen in J. Meier, A. Shah, "INTRINSIC MICROCRYSTALLINE (.mu. c-Si:H)-A PROMISING NEW THIN FILM SOLAR CELL MATERIAL," IEEE WCPEC; 1994 Hawaii, p.409. Further, a stacked solar cell was fabricated by combining of amorphous silicon with microcrystalline silicon and achieved an initial photoelectric conversion efficiency of 9.1%.
It is also known that a transparent, conductive layer is interposed between the substrate or metal layer and the semiconductor layers. This prevents elements in the metal layer from diffusing or migrating into the semiconductor layers, thus preventing the photoelectric conversion element from shunting. Further, it has a moderate resistance and prevents the semiconductor layers from short-circuiting due to a defect such as a pinhole. In addition, the transparent, conductive layer is provided with an uneven surface, thereby increasing irregular reflection of incident light and reflected light to lengthen optical pathlengths in the semiconductor layers.
With the above-stated solar cell using the microcrystalline silicon based material, however, the photoelectric conversion efficiency thereof is still too low, 4.6%, to be of practical use.
With the stacked solar cell of a-Si/.mu. c-Si, the initial photoelectric conversion efficiency is as high as 9.1%, but it suffers from great optical degradation of the a-Si layer on the light incidence side. Further, the thickness of the .mu. c-Si layer is thick, 3.6 .mu.m, and the rate of deposition is slow, 1.2 .ANG./sec. Thus, the time necessary for layer formation is approximately eight hours; this poses another problem in that the time for layer formation does not reach an industrially practical level.